Photovoltaic cells, such as solar cells, typically possess a preferential optical operating spectrum across/within which it is desired to have the photovoltaic cell operate. This may be because the design of the cell causes it to be relatively efficient in respect of, or to be relatively sensitive to, optical radiation within such a preferential spectrum. Alternatively, or additionally, it may be the case that optical radiation outside the operating spectrum is in some way detrimental to the operation of the cell.
For example, the efficiency of the photo-electrical conversion process of many photovoltaic cells falls as the temperature of the cell rises. Consequently, many solar cells employ filters designed to exclude from radiation incident upon the cell those wavelengths of radiation which have a propensity to heat the cell.
Typically, the filters are designed to reflect incident Infra-Red (IR) radiation. Other filters designed to exclude other types of optical radiation, such as Ultra-Violet (UV) radiation which may be damaging to certain components of the solar cell, may be employed. These other filters may be separate from the IR type filters. Alternatively, a single “band-pass” filter may be employed which is designed to simultaneously exclude not only IR radiation but also UV radiation from radiation incident upon a solar cell, but let “pass” through the filter optical radiation lying within a spectral band bounded by the excluded IR and UV radiations.
In general a silicon solar cell is operative responsive to radiation of wavelengths between 0.40 μm to 1.10 μm. Solar energy outside of this band is generally not converted into electricity and when absorbed only heats up the solar cell thereby reducing its efficiency. Certain types of optical band-pass filter comprise a multi-layer stack structure arranged upon a substrate, such as a glass substrate. Such multi-layer band-pass filters are designed to reflect IR radiation (or near-IR radiation) that lies immediately adjacent one side of a spectral band of radiation in respect of which a solar cell is intended to operate (e.g. 0.4 μm to 1.1 μm wavelengths), to transmit optical radiation lying within that spectral band and to reflect UV lying adjacent the other side of the spectral band of the solar cell.
The band-pass transmission spectra of such multi-layer filters is achieved by forming the stack of layers from repeating pairs of adjacent layers in which any one layer of a pair is comprised of material having an index of refraction which differs from the index of refraction of the other layer of the pair. Thus, the resulting multi-layer stack has an index of refraction which periodically jumps between two values as the depth of the multi-layer stack increases.
As is well known to those skilled in the art, the “optical thickness” of a layer is given by multiplying the physical thickness of the layer by the index of refraction of the material of the layer for a particular wavelength of optical radiation. Thus, a layer of constant physical thickness will have an optical thickness which depends upon the wavelength of optical radiation passing through it.
By appropriately controlling the physical thickness of each of the two layers in the repeated pair of layers such that each has an optical thickness equal to ¼ of a predetermined optical “design” wavelength (e.g. an IR wavelength), the multi-layer stack causes reflection of optical radiation to occur not only at (and around) the predetermined “design” wavelength, but also at (and around) other wavelengths corresponding to “higher order” frequencies equal to an odd-integer multiple of the “design” frequency. The result is known as a “¼-wave” stack, or “interference filter”.
Thus, such a quarter-wave stack may be employed as an optical filter for reflecting IR radiation by selecting the predetermined wavelength to be a suitable IR wavelength such that a reflection band is formed adjacent a desired spectral pass-band. However, a drawback of such interference filters is that the aforementioned “higher order” reflection bands often reside well within the desired spectral pass-band. Thus, such filters may well reflect radiation which it is not desirable to reflect.
Other types of multi-stack optical interference filter have been proposed in which the multi-layer stack is composed of materials having three different refractive indices rather than just two. An example of that sort of optical filter is described in U.S. Pat. No. 6,107,564. By appropriately arranging the three different layer types in a repeating pattern within the stack of layers, the resultant structure is able to suppress the first few of the offending “higher order” reflection bands which usually occur in simple ¼-wave stacks as discussed above.
However, a common feature of the simple two-index ¼-wave stacks and of the aforementioned three-index stacks is the presence of a discontinuity in refractive index as between neighbouring stack layers. This discontinuity arises due to the sudden change in material (and optical properties thereof) at the interface between adjacent stack layers. Such discontinuities are detrimental to the performance and structure of the filter for the following reasons.
The strength of the multi-layer stack is sensitively dependent on the degree of interfacial adhesion between adjacent stack layers. Since adjacent layers in the aforementioned prior art devices are comprised of different materials, it is often the case that the differences (either chemical and/or physical) between such neighbouring layers reduce the strength of the interfacial bond which results in the interface being a primary site of structural weakness in the multi-layer stack.
Furthermore, in existing interference filters such as those described above, the transmission spectrum thereof at regions in between successive reflection bands are not uniformly transmissive. That is to say, although the reflection bands of such interference filters are generally mainly confined to a limited spectral band, they are in fact not fully so confined. Rather, the so-called reflection “bands” often possess significant ripples or transients of non-zero optical reflectance in the filter at spectral regions within the pass-band thereof.
This spreading/dispersion of the reflection band into the pass-band is principally due to the discontinuity in refractive index occurring at the interface of successive layers of a stack. It is detrimental to the transmission spectrum of such filters since it attenuates optical radiation which it is intended to pass to an underlying solar cell. Hence, solar cell efficiency is reduced.